Various types of exposure systems are currently in use for imprinting micro-patterns onto the surfaces of substrates such as semiconductor wafers. A typical exposure system includes an illumination source, a first stage apparatus that holds and positions a pattern master (e.g., a reticle), a second stage apparatus (downstream of the first stage apparatus) that holds and positions the substrate, a projection-optical subsystem situated between the first and second stage apparatus, and a control subsystem connected to and exercising operational control over these apparatus and subsystems. Since the sizes of the elements of the exposed pattern are very small (currently in the several tens of nanometers), the first and second stage apparatus must be capable, as controlled by the control subsystem, of achieving extremely accurate and precise positioning of the stage apparatus and projection-exposure system relative to each other so as to achieve corresponding accuracy of exposure.
Substantially all exposure systems currently in use employ various sensors, detectors, and other measurement devices for determining and monitoring the accuracy and precision of stage position and of many other operations performed by the exposure system. For example, interferometers are widely used for sensing position of the wafer stage (also called substrate stage) during use of the system for making exposures. Other position-measuring devices in current use utilize motion encoders. During operation of the exposure system, these sensing devices collectively determine stage position in various degrees of freedom at very high accuracy and precision. These measurements are used in various ways, such as to “servo” stage motion and to provide stage positioning as required for accurate and precise exposures.
Measurements of position of the substrate stage obtained during system operation are typically relative measurements. Provision of reference data for a stage-position measurement is facilitated by the fact that, in any moment of time during operation, the system already “knows” the location of the stage. But, there are certain instances in which providing a reference location for stage location can be more difficult. An exemplary instance is during start-up of the exposure system.
Whenever an exposure system is started up, something usually must be done to “initialize” stage positions, i.e., determine an initial stage position relative to the projection-optical system to provide a baseline(s) for subsequent positioning movements and measurements of or involving the stage. This initialization requirement can apply to one or both stages in a microlithography system. Some conventional systems execute a start-up routine in which the stage is momentarily moved to a starting, or initialization, position at which starting-position data are obtained. Unfortunately, this routine consumes valuable time.
Position sensors used for determining the starting position of a stage may not be the same as the stage-position sensors used during system operation. Sensors used at startup, particularly those not requiring return of the stage to an initialization position, desirably provide absolute position data rather than relative position data. For example, for determining stage height during system operation, a monochromatic interferometer provides a relative measurement. Since phase is measured modulo 2π in phase, one interference fringe is indistinguishable from the next. This normally does not cause a problem during normal system operation but can pose a problem during system startup because an indistinguishable interference fringe does not provide a position reference. A similar difficulty exists with encoder-reading heads that operate using light diffracted from a grating on the stage; these devices are also, in effect, interferometers.
Another challenge posed particularly to an interferometric sensor for determining initial stage z-position is the possibility that the stage may not be, at startup, exactly perpendicular to the measurement beam of the interferometer. I.e., the stage may be exhibiting tip and/or tilt (θx, θy). This lack of perpendicularity can introduce significant error into the z-position determination.
Therefore, there is a need for accurate interferometric sensors useful for accurately determining stage position (e.g., stage “height” or z-position) as required, e.g., upon system startup, even in situations in which the measurement beam of the sensor is not exactly perpendicular to the stage.